1. Field of the Invention
The present invention relates to an oscillation control apparatus that performs oscillation control of a VCO (Voltage Controlled Oscillator) which is a configuration element of a PLL (Phase Locked Loop), and more particularly to an oscillation control apparatus that detects and automatically adjusts f/v sensitivity, f/v being the ratio of oscillation frequency f to VCO control voltage v.
2. Description of the Related Art
A conventional general oscillation control apparatus employs a control method whereby oscillation control is performed without automatically adjusting the f/v sensitivity of a VCO. See, for example, Non-Patent Document 1 (Transistor Technology (June 1996 issue, Vol. 36, No. 417, P230)).
FIG. 20 is a block diagram showing the configuration of a conventional oscillation control apparatus that has virtually the same configuration as in Non-Patent Document 1. This oscillation control apparatus comprises VCO 41 provided with resonator 41a and oscillator 41b, frequency divider 42, phase comparator 43, and low-pass filter (LPF) 44. Phase comparator 43 compares the phases of reference signal f0 and an output signal f1 of frequency divider 42 that divides the output frequency of VCO 41, and outputs output signal f2 in accordance with the phase difference to LPF 44. LPF 44 performs integration of output signal f2 from phase comparator 43 and eliminates an unnecessary high-frequency component, and outputs a control voltage of resonator 41a of VCO 41. Then oscillator 41b functions as a PLL by oscillating at the resonance frequency of resonator 41a based on that control voltage.
A technology has also been disclosed whereby the operating environment and manufacturing variation are identified by detecting the VCO control voltage, and the optimal oscillation frequency band and frequency divider division ratio are selected from the plurality of oscillation frequency bands with which the VCO is provided, according to that identification. See, for example, Patent Document 1 (Unexamined Japanese Patent Publication No. HEI10-271002).
However, with the above-described conventional technology, if the f/v sensitivity of the VCO varies due to configuration element variation, temperature changes, or the like, the C/N (Carrier to Noise ratio) may degrade, or the frequency variability range may be narrowed. FIG. 21 is a graph showing variation of f/v sensitivity in a conventional VCO, with input voltage (V) represented on the horizontal axis and output frequency (Hz) on the vertical axis. That is to say, this graph shows the degree of change of the frequency change width due to f/v sensitivity variation in the conventional VCO. In this graph, taking f/v sensitivity reference characteristic A as the optimal slope of the f/v sensitivity of the VCO, if the f/v sensitivity varies in the downward direction as shown by low f/v sensitivity characteristic B, the frequency variability range narrows, and if the f/v sensitivity varies in the upward direction as shown by high f/v sensitivity characteristic A, the C/N ratio of the VCO degrades.